Articles which have release surfaces are used in a variety of applications. Pressure sensitive adhesive articles, such as tapes, die cuts, labels and the like often employ a release coating as a part of their structure, the coating oftentimes being referred to as a low-adhesion backsize (LAB). The purpose of the release coating is to provide a surface to which the adhesive coating does not permanently adhere. This is especially useful when the article is wound upon itself in roll form. In this form, the adhesive surface contacts the back surface of the article. The LAB prevents the adhesive from permanently adhering to the back surface of the article and allows that article to be unwound.
In the past, release coatings have been applied to surfaces by dissolving the release components in solvent, coating the solution onto a desired surface, and drying to evaporate the solvent. These solvent-based processes have become increasingly undesirable because of the environmental and safety considerations associated with the use of solvents.
This has led to accelerated interest in the use of hot melt extrusion coating of release compositions. For example, U.S. Pat. No. 4,379,806 (Korpman) discloses preparation of pressure-sensitive adhesive tapes by a single step process of coextruding a backing film-forming composition and an adhesive composition. A solventless release agent may be incorporated into either the adhesive or backing, rendering unnecessary the coating of the backside of the tape with a release agent. Korpman states that suitable release agents for incorporation into backing film include synthetic and natural waxes and copolymers of stearyl methacrylate and acrylonitrile. U.S. Pat. No. 5,145,718 (Pedginski) discloses a pressure sensitive adhesive tape wherein a low-adhesion backsize and a pressure sensitive adhesive are applied to opposite sides of a backing that has been oriented in the machine direction. After application of the low-adhesion backsize and adhesive, the backing is oriented in the transverse direction by heating and cross-stretching in the transverse direction. Japanese patent SHO 581983!-113283 discloses a process for manufacturing a pressure-sensitive adhesive film comprising a substrate layer, pressure-sensitive adhesive layer and release layer wherein said pressure sensitive adhesive layer is the middle layer. The release layer may be either coextruded along with the other layers, or may be extruded separately and laminated to the adhesive layer of the two-layer film outside of the die.
Several references disclose coextrusion of a film backing and a silicone-containing release layer for pressure sensitive adhesive tape applications. For example, U.S. Pat. No. 4,673,611 (Crass) discloses a tape comprising a coextruded, biaxially-oriented multilayer polypropylene support film and an adhesive layer, wherein the support film comprises at least two layers, the layer facing away from the adhesive comprising an anti-adhesive substance containing polydiorganosiloxane. U.S. Pat. No. 4,839,123 (Duncan) discloses a process for making an oriented polymer film in which a solventless, curable silicone release composition is applied to an extrudable, orientable polymer. The combination of the silicone material and the polymer is extruded as a sheet or film. The orientation process causes the silicone release composition to migrate to at least one surface of the sheet or film. The release composition is then cured to form a substantially continuous release layer on the oriented sheet or film. EPO application 0-484-093-A2 (Oy) discloses a polymer composition for forming release films. The film is formed of a reactive polyolefin that contains functional groups and a reactive silicone or other reactive release substance. The ingredients are caused to react by compounding them together in a molten state. The reactive compounding can be performed in connection with the film preparation to form a release film. The film can be prepared by means of a blown-film, cast-film or extrusion coating technique. German patent application DE 37-10-670 A1 discloses a pressure sensitive adhesive tape with a hand tearable backing that may have an anti-adhesive layer comprising polypropylene with 0.2-3 wt % added polydiorganosiloxane. The backing and anti-adhesive layer may be coextruded. U.S. Pat. No. 5,213,743 (Ohara) discloses a release agent formed by reacting an organopolysiloxane with a hydrocarbon compound having at lease one double bond. The release agent is said to be suitable for both coating and extrusion processing. Japanese Kokai Hei 3-229,776 (Kitamura) discloses a method of making pressure sensitive adhesive tape by coextruding a base material, for instance, polyolefin resin, and a resin layer incorporating a release agent formed from an organic silicone compound. The silicone compound possesses at least one hydrogen group and a hydrocarbon having at least one double bond that is capable of reacting with the hydrogen bond of the silicone compound. The coextruded multilayer film may be oriented in at least one direction before coating with adhesive, or alternately the adhesive may be applied to the coextruded multilayer film and then the entire construction oriented to form a tape.
Silicone-based release coatings, whether solvent or extrusion coated, often show increased transfer with aging. As a result, when used on tapes that are stored in roll form, the release material often transfers to the adhesive surface. Fluoropolymer release coatings are generally more stable and do not show increased transfer with aging. U.S. Pat. No. 4,677,017 (DeAntonis) discloses a multilayered coextruded film and coextrusion process wherein the coextruded film comprises at least one thermoplastic fluoropolymer layer, at least one thermoplastic polymeric layer adjacent to the thermoplastic fluoropolymer layer, and preferably a coextruded adhesive layer between each fluoropolymer layer and each polymeric layer. Each layer in the coextruded film is at least 0.05 mils thick. The coextruded film can be embossed or oriented in at least one direction without delamination, fibrillating or splitting.
The release coated films and articles of the prior art have not proven to be entirely satisfactory. Oftentimes a tie layer is needed to sufficiently adhere the release layer to the film. For example, flame or corona treatment is often employed to enhance said adhesion. In other cases, the release coating migrates upon aging despite the use of the tie layer. In still other cases, limitations exist as to suitable backing materials that may be employed with a given release coating.
A release coated film is therefore needed in which the release layer is stable with aging and extrudable onto a variety of backing materials without the use of additional treatment or a tie layer to enhance the adhesion of the release layer to the backing film. Preferably, the release coated layer can be coextruded with the backing layer without the use of solvents. It is also desirable that the release layer and backing layer be selectable from a wide variety of polymers so that the release coated film characteristics may be optimized for a particular application.
The present invention provides a stable release coated film wherein no tie layer or additional treatment is employed to improve the adherence of the release coating to the substrate surface. The present invention further provides a release coated film which may be made without the use of solvent processing techniques. The present invention also provides an adhesive tape in which minimal release material transfers to the adhesive layer when the tape is stored in roll form. Additionally, the release coated film and backing materials can be varied to optimize the release coated film for specific applications.